Treatment fluids can be employed in a variety of subterranean operations. As used herein the terms “treatment,” “treating,” and grammatical equivalents thereof refer to any subterranean operation that uses a fluid in conjunction with performing a desired function and/or for achieving a desired purpose. Illustrative subterranean operations that can be performed using treatment fluids can include, for example, drilling operations, fracturing operations, sand control operations, gravel packing operations, acidizing operations, conformance control operations, fluid diversion operations, fluid blocking operations, and the like.
In many cases, treatment fluids can be utilized in a gelled state when performing a treatment operation. For example, in a fracturing operation, a treatment fluid can be gelled to increase its viscosity and improve its ability to carry a proppant or other particulate material. In other cases, a treatment fluid can be used to temporarily divert or block the flow of fluids within at least a portion of a subterranean formation. In the case of fracturing operations, the treatment fluid typically spends only a very short amount of time downhole before the gel is broken and the treatment fluid is produced from the wellbore. In fluid diversion or blocking operations, often the gel needs to remain in place only for a short amount of time while another treatment fluid is flowed elsewhere in the subterranean formation; however, it can sometimes become necessary to block the flow of fluids in the subterranean formation for a prolonged period of time, typically for at least about one day or more. In some cases, the period of time can be one to five days. In other cases, the period of time can be much longer, such as weeks. For example, it can sometimes be desirable to impede the flow of formation fluids for extended periods of time by introducing a kill pill or perforation pill into the subterranean formation to at least temporarily cease production. As used herein, the terms “kill pill” and “perforation pill” refer to a small amount of a treatment fluid introduced into a wellbore that blocks the ability of formation fluids to flow into the wellbore. Likewise, in fluid loss applications, it can sometimes be desirable to form a barrier within the wellbore that persists for an extended period of time. In fluid loss applications a fluid loss pill is introduced in the wellbore to impede the flow of fluids into the formation from the wellbore. In fluid loss pill, kill pill and perforation pill applications, high density brines can be particularly effective as a carrier fluid, since they can be formulated to form a highly viscous gel that blocks the flow of fluids within the wellbore by exerting hydrostatic pressure therein.
Treatment fluids typically remain in a stable gelled state only for a finite period of time before they break into lower viscosity fluids. In many cases, the decomposition of a gel can be accelerated by using a breaker, if a faster break is desired. For subterranean operations requiring extended downhole residence times, many treatment fluids can prove unsuitable since they can break before their intended downhole function is completed. The premature break of treatment fluids can be particularly problematic in high temperature subterranean formations (e.g., formations having a temperature of about 200° F. or above) where the elevated formation temperature decreases the gel stability and speeds gel decomposition. As subterranean operations are being conducted in deeper wellbores having ever higher formation temperatures, the issues with long-term gel stability are becoming increasingly encountered as existing gels are being pushed to their chemical and thermal stability limits. Premature breaking can be particularly problematic in high temperature applications of biopolymer-based gellable treatment fluids (e.g., guar- and cellulose-based treatment fluids and the like), where thermally induced chain scission and molecular weight loss can accelerate gel breaking.
Synthetic gellable polymers having increased thermal stability have sometimes been used in place of biopolymers to extend the working temperature range of gellable treatment fluids. One issue with synthetic gellable polymers is that they can sometimes become crosslinked too rapidly or become overly crosslinked during gelling. If crosslinking occurs too rapidly, downhole introduction of the gellable treatment fluids can be complicated due to high friction pressures as the gel becomes too thick to effectively pump before reaching its intended location. If the gel becomes overly crosslinked, the gel can be too viscous, difficult to break and sometimes exhibit excessive syneresis whereby carrier fluid is exuded from the gel.
As can be appreciated from the above, a treatment fluid composition suitable for a particular downhole use and particular formation conditions will not necessarily be suitable for different downhole uses and/or different formation conditions; thus, there are numerous problems and considerations in developing an appropriate treatment fluid composition to be used in fluid loss pill, kill pill and perforation pill applications.